Advances in microbe engineering for the production of biofuels, chemicals, and therapeutics have spurred investment in the production of a wide variety of commodities from biological sources (Zhang F, Rodriguez S, Keasling J D. 2011. Curr. Opin. Biotechnol. 22(6):775-83). Heterotrophic microbes comprise the vast majority of microorganisms currently utilized for product generation and require a carbohydrate source for carbon and energy that can account for a significant proportion (˜60%) of input costs (Pimentel D, Patzek T W. 2008. Ethanol production: energy and economic issues related to U.S. and Brazilian sugarcane biofuels. Springer, Amsterdam, Netherlands.). Such carbohydrate feedstocks are typically derived from agricultural crops, primarily sugarcane, sugar beet, and corn, although lignocellulosic materials are under extensive investigation as alternative feedstocks (Sims R, Taylor M. 2008. From 1st to 2nd generation biofuel technologies. IEA, Paris, France). While biologically produced fuels and chemicals hold the promise of increased sustainability and reduced CO2 footprints, current feedstock sources place biotechnological processes in competition with agricultural croplands and food markets. The development of biological alternatives to standard petroleum-based fuels and chemicals has therefore been criticized for its capacity to increase food cost and instabilities (Timilsina G R, Beghin J C, van der Mensbrugghe D, Mevel S. 2010. The impacts of biofuel targets on land-use change and food supply. The World Bank Development Research Group, Washington, D.C.). Indeed, in recent years, sugar prices have increased and fluctuated greatly in global food, driven in part by increased demands for biofuel production.
Photosynthetic microorganisms (cyanobacteria and algae) have been proposed as alternative sources for the creation of biofuel-like compounds or industrial feedstocks (Radakovits R, Jinkerson R E, Darzins A, Posewitz M C. 2010. Eukaryot. Cell 9:486-501), in part because they possess many advantages over traditional terrestrial plants with regard to targeted metabolite production. For example, the photosynthetic efficiency of cyanobacteria is up to an order of magnitude higher than that of plants (Zhu X G, Long S P, Ort D R. 2010. Annu. Rev. Plant Biol. 61:235-261) (Zhu X G, Long S P, Ort D R. 2008. Curr. Opin. Biotechnol. 19:153-159), and cyanobacteria do not require support tissues that further reduce productive output (e.g., roots/stems). Cyanobacteria are genetically tractable, allowing for rapid engineering and the selection of desirable strains. Finally, cyanobacteria are aquatic microbes with minimal nutritional requirements and can therefore be cultivated in locations that do not compete with traditional agricultural crops. While cyanobacteria and algae share many similar features in this context, the use of algal species for biofuel feedstocks has been explored in much greater detail, partly because of their relatively high lipid content (Sheehan J, Dunahay T, Benemann J, Roessler P. 1998. Look back at the U.S. Department of Energy's aquatic species program: biodiesel from algae. Close-out report NREL/TP-580-24190. National Renewable Energy Laboratory, Golden, Colo.), although many cyanobacterial species feature relative simplicity and higher growth rates.
Glycogen that accumulates in microorganisms can serve as a valuable feedstock for the production of chemicals and biofuels. Glycogen can be converted to ethanol or other chemicals, for example, through saccharification and fermentation processes (Aikawa et al. Energ Environ Sci 2013, 6:1844-1849) (Choi et al. Bioresour Technol 2010, 101:5330-5336) (Harun et al. Appl Energy 2011, 88:3464-3467) (Ho et al. Bioresour Technol 2013, 145:142-149) (Miranda et al. Bioresour Technol 2012, 104:342-348).
There is a need for microorganisms capable of producing high amounts of glycogen or other carbohydrates, particularly through photosynthetic processes.